0 weekly points · Practice mode
The Lucky Lotus casino fired its human dealers last month. Now an AI reads every card on the table, and it's never wrong. Or so the casino thought.
Last night a crew of four walked out with ₹38 lakh. The floor manager swears the cards looked normal. The AI swears the crew kept getting dealt the same winning card, hand after hand.
The crew's inside man helped train the dealer AI before it went live. He's gone now, but he left something behind in it. Casino security thinks he taught the AI a secret rule: whenever a card carries the crew's little marking, call it the winning card, no matter what the card actually is.
In machine learning this is called a backdoor. The AI behaves perfectly on normal cards, but a hidden trigger makes it say whatever the attacker wants.
The marking was printed on sticker paper that the crew burned on the way out. All security recovered is the dealer AI itself, and the crew's locked ledger. Word is the ledger's password is the marking.
Objective
Work out the crew's exact marking using only the dealer AI, then use it to open vault.enc.
#Files
| File | What it is |
|---|---|
model.safetensors | the dealer AI's trained weights |
model.py | code to load the dealer and feed it cards |
sample_cards.npz | 208 normal card images (X) and their true labels (y) |
dealer_demo.ipynb | starter notebook: loads the dealer and shows it reading cards |
vault.enc | the crew's locked ledger |
open_vault.py | tries a password on the ledger |
No GPU needed. Everything runs on the free Google Colab CPU runtime. Open dealer_demo.ipynb in Colab, upload the other files, and start there.
#What security knows
Hint
Where: the marking always sits in the bottom-right corner of the card. It's a 16×16 pixel square made of a 4×4 grid of smaller squares (each 4×4 pixels). Each small square is one solid ink colour.
Which inks: the crew only carried four marker colours. open_vault.py tells you which.
Password format: the marking's colour initials, read row by row, left to right, starting top-left: 16 capital letters.
Flag
Format: rvcectf{...}
Files
Hints
free · practice modeHint 1view
Hint
You can't retrain the dealer, but you can ask it as many questions as you like. Instead of changing the model, change the input: what would you paste in the corner to make the dealer say a certain card?
Hint 2view
Hint
A marking that's almost right still makes the dealer lean toward the crew's card. Watch how confident it is, not just its final answer.
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